Industrial processes often require measuring the level of liquid or other material in a tank. Many technologies are used for level measurement. With contact level measurement some part of the system, such as a probe, must contact the material being measured. With non-contact level measurement the level is measured without contacting the material to be measured. One example is non-contact ultrasound, which uses high-frequency sonic waves to detect level. Another example is use of high-frequency or microwave RF energy. Microwave measurement for level generally uses either pulsed or frequency modulated continuous wave (FMCW) signals to make product level measurements. This method is often referred to as through air radar. Through air radar has the advantage that it is non-contact and relatively insensitive to measurement errors from varying process pressure and temperature. Known radar process control instruments operate at frequency bands of approximately 6 Ghz or 24 Ghz.
A typical through air radar measurement instrument converts a high frequency electrical signal to an electromagnetic wave. An oscillator is used to create the high frequency electrical signal. An antenna, such as a waveguide or horn, is operatively associated with the oscillator. The waveguide and/or antenna converts the high frequency electrical signal into an electromagnetic wave that can be directed at a target, such as a liquid level surface. An ultra-high frequency (26 GHZ, for example) radiation beam propagates downward from the antenna, is reflected off the surface of the material being measured, and returns to the antenna where the signal is received. The product level is calculated from the total time of propagation of the beam.
In most cases, there is some liquid inside the tank. Normally, radar pulses from the measurement device make a single trip downward to the liquid surface, are reflected by this surface back toward the antenna, and the round trip travel time is an indication of the distance to the liquid surface. This time of flight is used to calculate the level in the tank. The process of sending out a radar pulse and “listening” for the echo is repeated millions of times per second. The transmission rate of repetitive radar pulses is the so-called “pulse repetition frequency” or PRF. Because the radar pulses are very short in duration, for best spatial resolution, it is advantageous to send pulses at the highest PRF possible to improve the detected radar signal-to-noise ratio. Practical radar systems transmit several million radar pulses per second.
Known level measurement instruments, such as in U.S. Pat. No. 6,626,038 use equivalent time sampling (ETS) which uses an expansion factor to effectively reduce the speed of the process to simplify analysis. This requires a high degree of coherence between the transmit and associated receive pulses. Known ETS instruments use a crystal controlled clock and precise delay generation circuitry to maintain a high degree of timing (delay) accuracy in the scanning system. The delay accuracy is assured by careful noise free design of the delay generation circuitry.
Problems can arise when attempting to measure the level in an empty or nearly empty tank. This is a common situation in practical applications. In these situations, all or most of the metal tank bottom is exposed. The tank bottom may be dished or conical in shape, making the tank bottom a very good radar pulse reflector. In fact, in many applications the tank bottom may be a better radar signal reflector than the liquid surface. The strong reflected pulse travels upward and can again be reflected by a metal tank top, which may also be dished, flat, etc., making a very good radar reflector. Radar pulses transmitted into such a scenario may well bounce off the top and bottom multiple times before dying out. These situations, where radar pulses make many round trips between the tank top and bottom, are referred to as “tank rattles”. Tank rattle can have a disastrous effect on pulse radar level measurement. If the “rattles” exists long enough so that they survive into the next repetitive PRF cycle, then the rattles that have made multiple trips can be confused with reflections that made only one such trip. The rattles can obliterate, and render useless, the detected radar signal needed to measure level. FIG. 7 of the drawings illustrates the effect of tank rattles in an empty tank. The signal trace should consist of a reference or fiducial pulse followed by a relatively flat baseline, then a large pulse indicating the tank bottom. Instead, the empty tank produces a busy and unusable waveform literally filled with “false echoes”. The false echoes are illustrated by the extraneous peaks in the waveform and are not actual targets, but rather are rattles. Such a waveform cannot infer any information as to what may be in the tank. It certainly cannot indicate that the tank is empty. To eliminate problems of tank rattle, it is necessary for the radar receiver to differentiate between echoes that are related to the most recent transmitted pulses, and those that are related to the pulse sent from an earlier PRF cycle.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.